Let $\mathcal P$, $\mathcal L$ and $\Pi$ be sets of points, straight lines, and planes respectively.

The **“lies on” relation** $R$ is an arbitrary relation $R\subseteq \mathcal P\times\mathcal L.$ We say that a point $A\in \mathcal P$ **lies on** a straight line $h\in \mathcal L$ if the ordered pair $(A,h)$ is contained in $R$ in set-theoretical sense, i.e. $(A,h)\in R.$

Similarly, the **“lies on” relation** $S\subseteq \mathcal P\times\Pi$ means that a point $A\in \mathcal P$ **lies on** a plane $\alpha \in \Pi$ if the ordered pair $(A,\alpha)\in S$ in set-theoretical sense.

- In undergraduate mathematics, many authors define the “lies on” relation by the contained relation. A point $A$ lies on $h$ if it is contained in $h$, i.e. $A\in h.$ Similarly, if $A\in\alpha$ then the point $A$ “lies on” the plane $\alpha.$ These definitions are the most descriptive and intuitive ones but not the only possible.
- The reader should be aware that in Hilbert’s original, general sense, the relations $R$ and $S$ are arbitrary. All that counts is the set-theoretical “being contained” of the ordered pairs $(A,h)\in R,$ respectively $(A,\alpha)\in S.$ Nevertheless, Hilbert called his relation “lies upon” for the sake of a better intuition.

| | | | | created: 2019-12-21 05:28:01 | modified: 2019-12-21 06:36:00 | by: *bookofproofs* | references: [6260], [8231], [8251], [8324]

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[8231] **Berchtold, Florian**: “Geometrie”, Springer Spektrum, 2017

[8324] **Hilbert, David**: “Grundlagen der Geometrie”, Leipzig, B.G. Teubner, 1903

[8251] **Klotzek, B.**: “Geometrie”, Studienbücherei, 1971

[6260] **Lee, John M.**: “Axiomatic Geometry”, AMC, 2013